I mounted a momentary contact push button in the door frame on the hinge side. When the door is closed, the switch is pressed. Opening the door releases the switch.
You can see the switch location in the picture. There are actually two switches there now. One is for the baggage area lights; the other is wired in series with the cabin doors, for a door open message on the EFIS.
Both switches are working quite well with over 300 hours on the aircraft...
I used the same reed switch van's provides for the main doors. I used cushion clamps and mounted the magnet in the door, and then the switch in the frame and wired it to the G3X to alert me when it's open.
And just because I could, I have it activate the baggage and interior lights when I open it through a cube relay. It was pretty easy in implementation that's pretty clean when done.
These are the only photos I could find, but hopefully you get the idea.